The present invention provides a method to measure the cylinder pressure at a high resolution during the combustion event, while measuring the cylinder pressure at relatively low resolution during other portions of the engine cycle that are less critical to controlling the combustion process.
The calculation of cylinder pressure combustion-related data for control of both diesel and gasoline internal combustion engines has been of interest. In general, calculation of such data is based, at least in part, upon measurements of cylinder pressure by cylinder pressure sensors. The cylinder combustion wave form captured by such sensors is used to calculate engine operating parameters such as Indicated Mean Effective Pressure (IMEP), Start of Combustion, Location of Peak Pressure (LPP), etc. These parameters are then used to enhance closed-loop engine control. In an effort to provide accurate cylinder combustion parameter calculation results, each analog to digital (A/D) conversion of the sensor must be precisely aligned with the corresponding angular engine position. Cylinder pressure sensors providing data sampling with a one degree or better angular resolution may be required for accurate reproduction of the combustion wave form and required accuracy in the calculation of combustion control parameters.
In one known arrangement, voltage from the cylinder pressure sensor is fed to an A/D converter where the engine reference pulse (from the engine crank sensor) is used to trigger an A/D conversion of the sensor output at specific known engine angle references. One disadvantage of such an arrangement is that the angular resolution of available automotive engine encoders is relatively low, typically six degrees or more. This results in an angular sampling of the cylinder pressure sensor every six degrees when a sampling resolution of one degree or less is desired for more accurate cylinder pressure combustion calculations.
In an effort to overcome this problem, interpolation algorithms to generate a high-resolution encoder or reference pulse from a low-resolution reference pulse may be used. However, the interpolation function is most accurate at a constant engine speed. Furthermore, specific measurements such as Location of Peak Pressure (LPP) require a higher sample rate. Also, because engine speed is constantly changing, this method must take into account errors due to acceleration and deceleration in order to maintain the desired measurement accuracy. In general, this method tends to be software intensive.
Another solution that has been attempted in an effort to obtain higher engine resolution is to use an engine encoder capable of indicating the required resolution (360 degrees or greater) per engine revolution. However, this approach is more expensive than desired, and still limits the angular resolution to a fixed value.
Accordingly, a way to alleviate the problems associated with known pressure sensing arrangements would be beneficial.